1. Field of the Invention
This invention relates to the area of activated carbons. More particularly, this invention provides a mesoporous carbon material in the form of filaments having a high surface area. This invention also provides a process for the preparation of activatd mesoporous carbon material.
2. Description of Related Art
In recent years, research on porous carbon materials has been very active. The forms of porous materials include the conventional activated carbon bulk [Dubinin et al., 1993, Adsorption Science Tech., vol 10:17-26; Chiang et al., 1995, Toxicological Environmental Chem., vol 47:97-108; Takeuchi and Itoh, 1993, Sep. Technol., vol 3:168-175], activated carbon fibers [Alcaniz-Monge et al., 1994, Carbon, vol 32:1277-1283; Oya et al., 1996, Carbon, vol 34:53-57], fine carbon particles [Ghosal et al., 1995, Materials Research Society Symp. Proc., vol 371:413-423], and carbon aerogels [Fung et al., 1993, Materials Res., vol 8:1875-1885], which are a solid network of carbon atoms strung along various directions thereby enclosing pores. Activated carbon materials are used not only as adsorbents for purification and chemical processing, but also as catalytic materials, battery electrode materials and biomedical engineering materials. Activated carbon materials are obtained by increasing the porosity of carbon.
According to the International Union of Pure and Applied Chemistry, pores are classified into four types, namely macropores (diameter&gt;500 .ANG.), mesopores (20 .ANG.&lt;diameter &lt;500 .ANG.), micropores (8 .ANG.&lt;diameter&lt;20 .ANG.) and micro-micropores (diameter&lt;8 .ANG.). In conventional activated carbons, most pores are micropores. The pores in activated carbon fibers (including pitch-based, polyacrylonitrile-based and rayon-based carbon fibers) are mainly micropores with some micro-micropores. Carbon aerogels mainly contain mesopores with some micropores. Thus although carbon aerogels contain mesopores, the specific surface area of carbon aerogels is generally about 650 m.sup.2 g which is relatively low compared to activated carbons having specific surface areas as high as about 3000 m.sup.2 /g.
One of the requirements of porous activated carbons in biomedical engineering and other applications is that the activated material have mesopores and/or macropores. Many macromolecules and ions encountered in catalysis and batteries cannot penetrate the surface of carbons devoid of large percentages of such pores. A problem often encountered is how to fabricate a porous carbon material with a high specific surface area and good mechanical and physical properties.
U.S. Pat. No. 5,238,470 to Tolles et al. and U.S. Pat. No. 5,304,527 to Dimitri describe a method for producing highly activated and highly densified mesoporous carbon particles from lignocellulosic carbonaceous material. The patents also describe a method in which a carbonaceous material is treated with an activating agent to produce a plasticated carbonaceous material which is then densified to produce the activated carbon.
U.S. Pat. No. 5,242,879 to Abe et al. describes active carbon materials having an average pore radius of about 15 .ANG. to 30 .ANG., prepared from waste material.
U.S. Pat. No. 4,439,349 to Everett et al. describes mesoporous carbon wherein the interparticle distances are of mesoporous size.
U.S. Pat. No. 5,472,742 to Lee describes activated carbon fibers produced by exposure to graphite oxide former followed by exposure to CO.sub.2 former.
While activated carbon particles are useful in many applications, the filamentous form of activated carbon according to the present invention is preferred in some applications. Filamentous activated carbon like cotton wool clings together to form a mat without the need for a binder. The mat structure facilitates handling, sandwiching, rolling and cutting. Thus filamentous activated carbon according to the present invention is much more processeable than activated particles which require a binder and cannot be easily made into a thin flexible sheet. Further, a mat or compact of the present filamentous activated carbon resembling cotton wool has macropores between adjacent filaments. The macropores enhance the hydrodynamics of the mat as they provide channels for the flow of fluids thereby enhancing process kinetics.